Google Apps Script for creating collapsible groups (assumes data is sorted, requires underscore)

group_by_column.gs

function run() {
  groupByColumn(0);
  groupByColumn(1);
  groupByColumn(2);
}

function selectRows(sheet, startRow, endRow) {
  let numRows = endRow - startRow;
  let startRow1Indexed = startRow + 2;
  return sheet.getRange(startRow1Indexed, 1, numRows, sheet.getDataRange().getNumColumns());
}

function groupByColumnB() {
  return groupByColumn(1);
}

function groupByColumn(columnIndex) {
  let sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
  let range = sheet.getDataRange();
  let values = range.getValues().map((row, index) => ({ row, index }))
  values = values.slice(1); // skip header row
  let colValues = values.map(({index, row}) => ({index, col: row[columnIndex]}))
  let groups = _.groupBy(colValues, 'col')
  for (let [group, rows] of Object.entries(groups)) {
    console.log({group, rows})
    rows = rows.map(row => row.index)
    if (rows.length > 2) {
      selectRows(sheet, _.min(rows), _.max(rows)).shiftRowGroupDepth(1)
    }
  }
}